Arsenic is one of the most toxic elements and is a known human carcinogen. Because of high toxicity associated with arsenic, the industrial uses of arsenic have been drastically curtailed in recent times thereby reducing the quantum of arsenic waste generation. However, the historic operations have lead to generation of significant quantities of arsenic wastes, which are still stored at many production facilities, where arsenic was used as one of the raw materials. The most significant among such production facilities are nitrogenous fertilizer-manufacturing complexes, wherein the well-known Giammarco-Vetrocoke process was predominantly employed during the manufacture of ammonia from feedstock (such as natural gas and naphtha). In the Giammarco-Vetrocoke process, carbon dioxide is removed from ammonia stream by scrubbing with a proprietary solution containing potassium carbonate, arsenic and other activators. However, due to several operational problems, and very high toxicity associated with arsenic, most of the nitrogenous fertilizer manufacturing complexes have switched over from the Giammarco-Vetrocoke process to other non-arsenic based processes since last one decade. The spent absorbing solution from the Giammarco-Vetrocoke process containing very high concentration of arsenic (up to 10%) was discarded as hazardous waste. Such wastes may be stored at many nitrogenous fertilizer-manufacturing complexes either in liquid form or in solid/semi-solid form.
The other industrial sources of arsenic wastes include production of pesticides, herbicides, or veterinary pharmaceuticals, wood preserving operations, non-ferrous metallurgical industries (such as copper smelters) and development of semiconductor material for the electronics industry. The liquid and solid wastes generated from these sources also contain very high concentration of arsenic.
Since recycling of arsenic-containing materials is technically challenging and cost prohibitive, there is a great demand for the development of effective treatment technologies for the safe disposal of arsenic contaminated hazardous wastes. With the advent of stringent environmental laws and the realization that arsenic containing hazardous wastes can cause severe damage to human, plant and animal life, many methods have been proposed for treatment and disposal of arsenic in various environmental media.
Reference may be made to the report EPA-542-02-004 (“Arsenic Treatment Technologies for Soil, Wastes and Water”, EPA-542-02-004, September 2002) by United States Environmental Protection Agency (USEPA), wherein an exhaustive review of arsenic treatment technologies applicable to water and wastewater have been presented. The treatment technologies for water and wastewater include precipitation/co-precipitation, membrane filtration, adsorption, ion exchange, and permeable reactive barriers. However, the maximum initial concentration of arsenic treated by these methods was 3,300 mg/L. Further, most of these methods generate a solid/semi-solid residue or liquids containing high concentrations of arsenic which are further required to be treated and disposed off.
Reference may again be made to the report EPA-542-02-004, (“Arsenic Treatment Technologies for Soil, Wastes and Water”, EPA-542-02-004, September, 2002) by the United States Environmental Protection Agency (USEPA), wherein an exhaustive review of arsenic treatment technologies applicable to arsenic wastes have been presented. The treatment technologies for arsenic wastes include solidification and stabilization, vitrification, acid extraction for arsenic, and pyro-metallurgical recovery for arsenic. Among these technologies, the most effective, least expensive and frequently used technology for arsenic wastes is solidification and stabilization. The solidification/stabilization technology generates a solidified mass that does not require further treatment for disposal. The review indicates that solidification/stabilization technology have been applied to the solids/semisolid wastes having high arsenic concentrations (up to 750,000 mg/kg) but having relatively low leachability [up to 4,390 mg/L as determined by USEPA's Toxicity Characteristics Leaching Procedure (TCLP)]. Most of the wastes mentioned in the review were generated during the treatment of liquid wastes containing high concentration of arsenic. The transformation of arsenic form liquid phase (by precipitation/co-precipitation, adsorption, ion exchange) to solid/semi-solid phase (sludges) and further treatment of arsenic in solid/semi-solid phase by solidification and stabilization involve two stages, thereby increasing the overall cost of treatment. No attempts have been made in the past to directly treat the liquid wastes (such as the spent absorbing solution from the Giammarco-Vetrocoke process) containing very high concentration of arsenic (up to 100,000 mg/L) by solidification solidification/stabilization. The direct treatment of liquid wastes containing very high concentration of arsenic by solidification/stabilization will eliminate the additional stage of converting liquid wastes to solid or semi-solid form, thereby significant reduction in overall cost associated with arsenic treatment.
Reference may also be made to the U.S. Pat. No. 5,098,612 wherein a method of preparing solidified and stabilized hazardous or radioactive liquids by direct treatment with solidification and stabilization was presented. However, the prime objective of this method was to reduce the dimensional expansion of solid mass during the solidification. The method involves addition of solidifying composition comprising a mixture of clay (selected from the group consisting of sodium montmorillonite, attapulgite, sepiolite, and mixtures thereof), and the silicone coated cementitious material, to the aqueous liquid or sludge in appropriate ratios to produce an unpourable, free standing solid mass. Though the method was applied to a wide variety of liquid wastes (containing organics, inorganics, metals and radioactive substances), the maximum concentration of contaminants in general, and arsenic in particular, was not discussed. Further, there is no discussion on the leachability of contaminants from the solidified and stabilized mass obtained by this method. The solidified and stabilized wastes must meet the USEPA TCLP criteria prior to the final disposal, such as landfills. In this context reference may also be made to a review by Liest et al., (Journal of Hazardous Materials B76, 2000, 125-138) wherein a number of solidification/solidification processes for treatment of arsenic have been presented. These include fixation of arsenic with various combinations of Portland cement, iron, lime, fly ash, slags and silicates. Since arsenic is present in both trivalent as well as pentavalent form, the trivalent arsenic, which is highly soluble in water, needs to be converted to an insoluble form such as calcium or ferric arsenate. Thus, use of cement and/or clay alone will not effectively solidify and stabilize the liquid wastes containing high concentrations of arsenic.
The reference may also be made to the study by Palfy et al. (P. Palfy, E. Vircikova, L. Molnar; “Processing of arsenic wastes by precipitation and solidification”, Waste Management, 19, 55-59, 1999) wherein a process for solidification and stabilization of arsenic sludge accumulated in the reaction tower during the refining of carbon dioxide in the Giammarco-Vetrocoke process was presented. Though, the arsenic waste studied by Palfy et al., 1999 contain high concentration of arsenic (163,000 mg/kg), the concentration of arsenic in the leachate is only 6,430 mg/L.
The reference may also be made to the presentation by Shields et al., 2001 (P. J. Shields, S. Nagaraja, L. D. Fiedler; “Treatment Technologies for Wastes and Environmental Media Containing Arsenic” U.S. EPA Arsenic Workshop. May 1-3, 2001, Denver, Colo.) Wherein a summary of treatment performance of solidification/stabilization for arsenic wastes was discussed. Though, the concentration of arsenic in the wastes was 750,000 mg/kg, the maximum leachable concentration of arsenic was only 100 mg/L.
Thus, the solidification/stabilization method discussed by Palfy et al., 1999 and Shields et al., 2001 is not applicable directly to the liquid wastes containing high concentration of arsenic (up to 100,000 mg/L).
The widely practiced, conventional method for treatment of liquid wastes containing very high concentration of arsenic involve transformation of arsenic form liquid phase (by precipitation/co-precipitation, adsorption, ion exchange etc.) to solid/semi-solid phase (sludges) and immobilization of arsenic in solid/semi-solid phase by solidification and stabilization. The major drawback of this method is the involvement of an additional stage of transforming arsenic from liquid phase to solid phase prior to solidification/stabilization, thereby increasing the overall cost of treatment of liquid arsenic wastes. The present invention thus deals with direct solidification and stabilization of liquid wastes containing very high concentration of arsenic, wherein an intermediate stage of transforming arsenic from liquid phase to solid/semi-solid phase is eliminated.